Simple Shell - Minimal Unix Command Interpreter

Simple Shell

A lightweight Unix-like command interpreter implemented in C, designed to mirror the essential behavior of early Unix shells while remaining intentionally minimal. The project focuses on core process control, parsing, and environment management, providing a clean foundation for understanding how shells work under the hood.

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Overview

Simple Shell reads user input, tokenizes commands, resolves executables using the PATH, and executes programs in child processes. It supports common shell behaviors such as chaining, comments, and aliases, alongside a practical set of built-in commands for environment and session control.

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Key Capabilities

• Command execution: runs external programs via fork() + execve()
• PATH resolution: locates executables using the PATH environment variable
• Builtins: cd, exit, env, setenv, unsetenv, help, history
• Chaining: supports ;, &&, and ||
• Comments: ignores content after #
• Aliases: define and expand custom command aliases
• Variable replacement: expands environment variables in input

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Architecture Highlights

• Parser and tokenizer: handle command splitting and argument extraction
• Execution engine: dispatches builtins or forks external commands
• Environment layer: maintains a linked-list representation of environ
• History manager: persists and reloads command history

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Usage

Compile

gcc -Wall -Werror -Wextra -pedantic -std=gnu89 *.c -o simple_shell

Run (interactive)

./simple_shell

Run (non-interactive)

echo "ls -l; pwd" | ./simple_shell

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Example Session

$ ./simple_shell
simple_shell$ ls
file1.c  file2.c  README.md
simple_shell$ cd ..
simple_shell$ pwd
/home/user/projects
simple_shell$ setenv MYVAR Hello
simple_shell$ echo $MYVAR
Hello
simple_shell$ exit

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Requirements

• Ubuntu 20.04 LTS (or compatible Linux)
• GNU Compiler Collection (gcc)
• Standard POSIX system calls

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Design Goals

• Minimal and readable codebase for learning purposes
• Accurate core shell behavior without unnecessary complexity
• Portable and standards-based (GNU89 + POSIX)

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License

Open source; free to use and modify.